The terms "dual-phase steel" are used to refer to a steel consisting essentially of a dispersion of martensite in a fine-grained ferrite matrix; these steels are of increasing technological interest because they provide a better combination of strength and ductility than any other steel sheet material. The strength is important because it offers the opportunity to produce lighter components, while good ductility is needed to permit the easy forming of components. It is generally recognized that martensite plays a role in determining the strength level of dual-phase steels. A full understanding of the role played by the phases in determining such characteristics needs to be more fully explored. How to maximize the martensite level of such steels while maintaining good ductility remains the subject of continued research in the industry.
There are currently two methods used to commercially produce dual-phase steels in the United States: (a) inter-critical annealing or (b) austenitic treatment. Inter-critical annealing requires the use of an annealing cycle wherein the steel is heated to the alpha+gamma phase region (approximately 730.degree.-840.degree. C. at 0.1 weight percent carbon) and is then cooled at a rate of less than 100.degree. C. per second. The austenite treatment involves controlling the cooling of the steel in the austenitized condition during hot rolling so that the cooling rate is adjusted to promote the existence of both ferrite and martensite in the cooled product.
The chemical composition of the steel used to produce such dual-phase steels by either method typically comprises 0.1% carbon, 1.4% manganese, 0.5% silicon, along with a singular addition of either 0.1% or 0.15% molybdenum or 0.4% chromium.
It has been discovered, with respect to this invention that the strength of dual-phased steels is controlled essentially by the amount of martensite in the structure and that this relationship is linear. It has been further observed as part of this invention that not all of the austenite transforms to martensite upon cooling by either of the methods above described. With intercritical annealing according to the prior art, one finds that additional ferrite and carbides are formed without a complete conversion of all of the austenite to martensite. With respect to the austenitized dual-phase steels, a considerable amount of carbides are formed because the slow cooling path enters the region in which carbides are formed. Therefore, there remains the technological problem of how to achieve an increased martensite content in a dual-phase steel without generation of carbides and to do so at a cooling rate which is less than 100.degree. C. per second, a limitation imposed by the available equipment used in most steel mills today. It is well recognized that if cooling rates highly in excess of 100.degree. C./sec., such as 1000.degree. C./sec., were employed, there would be no difficulty in obtaining increased austenite conversion to martensite; but this alternative is not economically satisfactory.